Electrical connectors, such as ball-grid array (BGA) connectors, are commonly mounted on a substrate using multiple solder connections. The solder connections act as electrical and mechanical connections between the substrate and the connector.
The connector and substrate typically operate at temperatures above ambient. Temperature changes can cause the connector and substrate to deflect, i.e., to expand or contract. (The amount of deflection of a component as a function of temperature change often is expressed as the coefficient of thermal expansion (CTE) for the component.) The amount of deflection experienced by the connector and substrate in response to a given temperature change usually differs. In other words, the CTEs of the connector and the substrate are usually different.
Differences between the amount of thermally-induced deflection of the connector and the substrate can induce stresses on the solder connections between the two components. These stresses, repeated over multiple heating and cooling cycles (referred to as “thermal cycling”) can weaken the solder connections. Weakening of a solder connection can affect the integrity of the signal transmission through the solder connection, and in extreme cases can result in separation of the solder connection from the connector or the substrate.
Positive temperature changes typically cause connectors to expand outward from the center thereof. Hence, the greatest amount of deflection in a square or rectangular connector occurs at and near its outer corners. Moreover, it is believed that the greatest differences between the respective deflections of the connector and the underlying substrate occur at and near the outer corners of the connector. The solder connections associated with these locations therefore are subject to relatively high thermally-induced stresses.